7. For a matter dominated universe, rho scales as a^-3, whilst the curvature term on the rhs of the Friedmann equation scales as a^-2. At early times (small a) the second term is negligible, and the universe decelerates as in a k=0 universe (so that a increases as t^2/3). At late times (large a), the curvature term takes over. Since k is -ve, the Friedmann equation tends towards da/dt=sqrt(kc²) - i.e. a constant velocity expansion.

The evolution of a(t) therefore looks something like this: